When welding the objects of welding such as thick steel plates by arc welding with a V-shaped or X-shaped groove formed between the objects of welding, the large space in the groove requires a large heat input. This causes such problems as the decrease in toughness of the welding heat affected zone.
To solve the above-mentioned problem, a conceivable method is to form an I-shaped groove, i.e., to form a narrow groove having groove faces facing each other in parallel with a narrow width. By this method, however, it is impossible to cause the arc to reach sufficiently to every corner of the groove to achieve satisfactory penetration into corners and to form a sound welding bead.
As a conventional method to solve the above-mentioned problem, a method is known which comprises carrying out a welding while causing oscillation of the tip of a welding electrode (hereinafter simply referred to as an "electrode") inserted into said narrow groove at a certain frequency in the width direction of the groove.
The above-mentioned welding method is however problematic in: that variation in the bending deformation of the electrode causes variation in the distance between the tip of electrode and the groove, making it impossible to obtain a stable arc; that a complicated oscillation mechanism of electrode is required; that, unless an oscillation width is strictly set up in response to the groove width, an excessive penetration into the objects of welding is caused by the electrode becoming too close to the groove face, and this may cause undercut or contact of the electrode with the groove face, resulting in short circuit of welding electrode; and furthermore, that a defective fusion may be caused at the lap portion of two adjacent beads because of the limited frequency of oscillation of the electrode.
The inventors therefore proposed a rotary arc-welding method, as a welding method which can solve the above-mentioned problems, disclosed in Japanese Patent Provisional Publication No. 133,871/80 dated Oct. 18, 1980 (hereinafter referred to as the "prior art"), which comprises the following steps.
An embodiment of the prior art is described with reference to FIG. 1.
In FIG. 1, 1 and 1' are the objects of welding; 2 is an I-shaped groove; and, 3 is a nozzle inserted vertically into the groove 2. Said nozzle 3 is rotatably fitted to a holder 4 fixed to the leading end of an arm described later around the axis thereof. Also in FIG. 1, 5 is a gear fixed to the top portion of the nozzle 3; 6 is another gear engaging with the gear 5, said gear 6 being rotated by a motor 7 fixed onto said arm; 8 is a tip fixed to the lower surface of the nozzle 3 at a position eccentric from the center axial line of the nozzle 3; 9 is a consumable electrode fed continuously from the end of the tip 8 through the nozzle 3 and the tip 8 toward the groove 2 by a feed roller 10; 11 are rails laid in parallel with the weld line on one of the objects of welding 1'; 12 is a carriage travelling on the rails 11 by means of wheels (not shown); 13 is a vertical screw stock fitted vertically to the carriage 12 through a fitting member 14; 15 is a motor, fixed to the top portion of the fitting member 14, for rotating the screw stock 13; and, 16 is an arm with one end screw-connected with the screw stock 13 and the other end fixed to a side wall of the holder 4.
A welding electric power source (not shown) is connected between said objects of welding 1 and 1' and said electrode 9. Welding current may be supplied to the electrode 9 either through the feed roller 10 or through the nozzle 3.
Another nozzle (not shown) for blowing an inert gas is provided near the tip portion of the nozzle 3 to permit ejection of the inert gas toward the weld zone during welding. When a bore for blowing inert gas is provided at the tip of said nozzle 3, it is not necessary to provide said nozzle for blowing inert gas.
Welding is carried out by setting the center axial line of the nozzle 3 at the center in the width direction of the groove 2 (hereinafter simply referred to as the center of the groove 2), and moving the carriage 12 while rotating the nozzle 3 through the gears 5 and 6 driven by the motor 7. When welding of a first layer has thus been completed, welding of a second layer is conducted after raising the nozzle 3 by a prescribed height through the screw stock 13 and the arm 16 by driving the motor 15. The above-mentioned operations are thus repeated to complete welding of the objects of welding 1 and 1'.
Since the tip 8 which guides the leading end of the electrode 9 is fixed at a position eccentric from the center axial line of the nozzle 3, an arc produced between the leading end of the electrode 9 and the objects of welding 1 and 1' follows a horizontal circular movement within the groove 2.
A wide arc having a certain width is thus obtained and reaches sufficiently to the groove corners. In addition, there is no risk of the electrode 9 coming too close to the groove face or coming into contact with the groove face, thus providing such useful effects as the formation of a sound bead and a higher welding efficiency.
The above-mentioned prior art has however the following problems.
More particularly, when the rails 11 laid on the object of welding 1' are not perfectly in parallel with the weld line, or when the groove 2 curves in the middle thereof although the rails 11 are laid straight, the center axial line of the electrode 9 (the center of rotation of the arc in circular movement) is not located at the center of the groove 2, and the arc in circular movement deviates toward either of the groove faces. As a result, the amount of penetration into the objects of welding 1 and 1' becomes non-constant, thus preventing a uniform bead from being formed.
When the object of welding 1' on which the rails 11 are laid curves up and down, the nozzle 3 moving up and down accordingly causes variation in the distance between the tip of the electrode 9 and the weld zone. As a result, the amount of penetration at the weld zone becomes non-constant, thus preventing a uniform bead from being formed.
Now, a change in welding current with rotation of the nozzle is described for the cases where the center of nozzle rotation is and is not located at the center of the groove.
As shown in FIG. 2(A), when the center axial line "a" of the nozzle 3 is located at the center "b" of the groove 2, the tip of the electrode 9 moves in a circle at the width center of the groove 2, as shown in FIG. 2(B). Along with rotation of the electrode 9, welding current varies as shown in FIG. 2(C). Suppose that the value of electric current when the tip of the electrode 9 is located at the center "b" of the groove 2 is "I.sub.o ", and the points at which said tip is located on a vertical plane in parallel with the welding direction are "a'" and "a"", then, welding current becomes larger according as said tip comes closer to one of the groove faces 2', takes a maximum value when said tip is closest to one of the groove faces 2', and becomes smaller according as the tip is then separated from said groove face 2' to become "I.sub.o " at position "a"". Then, welding current becomes larger again to reach the maximum value again when the tip becomes closest to the other groove face 2". Welding current thus takes the form of a current varying with a certain amplitude.
As shown in FIG. 3(A), on the other hand, when the center axial line "a" of the nozzle 3 deviates from the center "b" of the groove 2 toward the object of welding 1, the tip of the electrode 9 moves in a circle on a location eccentric toward one of the groove faces 2', as shown in FIG. 3(B). Rotation of the electrode 9 causes change in welding current as shown in FIG. 3(C). Said welding current becomes larger according as the tip of the electrode 9 comes closer to one of the groove faces 2', takes a maximum value when said tip is closest to one of the groove faces 2', and becomes smaller according as the tip is then separated from said groove face 2', to a value slightly larger than "I.sub.o " at position "a"". Welding current becomes "I.sub.o " when said tip comes to the center "b" of the groove 2, and becomes larger according as the tip becomes closer to the groove face 2", but does not take said maximum value even when the tip becomes closest to the groove face 2", but a considerably smaller current value. Thus, welding current becomes a current of which the amplitude varies every half a cycle.
As mentioned above, variation in welding current with rotation of the nozzle 3 causes the penetration into one object of welding 1 to become excessive as compared with the penetration into another object of welding 1', thus preventing a uniform bead from being formed.